From US patent application US 2002/0190595 A1, a line start electric motor is known wherein a start capacitor and a PTC-thermistor are connected in parallel with a run capacitor and in series with a start winding. The PTC-thermistor serves to separate the voltage source from the start capacitor when the line start electric motor reaches a given rotational speed. In the patent application it is mentioned that the PTC-thermistor can be replaced by a centrifugal force switch. Such a centrifugal force switch requires a relatively high construction expense. Moreover, the movable parts of the centrifugal force switch must be lubricated in operation in order to assure a disturbance-free continuous operation. The lubricating means can be injured by the electric contacts, for example, as a result of being burned by electric arcs.
From U.S. Pat. No. 6,356,047 B1, a starter for an electric motor is known in which a PTC-thermistor and a relay are connected in parallel with a run capacitor. When the motor is started, the relay closes and the PTC-thermistor is subjected to current flowing through it. During the initial motor rotations, the PTC-thermistor heats itself so that the current flow is reduced. When the motor has reached the desired rotational speed, the relay is opened again. The circuitry known from the American patent is not suited for a line start electric motor, since no start capacitor is provided.